Method for producing seamless steel pipe for inflator of air bag

ABSTRACT

The invention proposes a method for manufacturing a seamless steel pipe having high strength, high toughness, and high formability for an airbag. A method for manufacturing a seamless steel pipe comprising a process for manufacturing a raw material for steel pipe having a composition containing 0.01 to 0.10% of C, 0.5% or less of Si, 0.10 to 2.00% of Mn, more than 1.0% and 2.0% or less of Cr, and 0.5% or less of Mo to form the seamless steel pipe, a process for drawing the seamless steel pipe in cold working, and a quenching and tempering process for heating the seamless steel pipe to A c3  transformation point or more and 1050° C. or less and then quenching, then tempering at 450° C. or more and A c1  transformation point or less is proposed. A method for manufacturing a seamless steel pipe where the cold drawing process is replaced after the quenching and tempering process is proposed. The composition may further contain, in addition to the composition, one or two or more selected from 1.0% or less of Cu, 1.0% or less of Ni, 0.10% or less of Nb, 0.10% or less of V, 0.10% or less of Ti, and 0.005% or less of B.

TECHNICAL FIELD

This invention relates to a high-strength seamless steel pipe,particularly relates to a high-strength seamless steel pipe havingexcellent toughness and formability suitable for an airbag inflator.

BACKGROUND ART

Recently, improvement of safety against automobile collision has beendesired earnestly, particularly, a safety device for protecting a crewat collision is actively introduced. In particular, the airbag, which isdesigned to expand between the crew and a steering wheel or aninstrument panel, and absorb kinetic energy of the crew, thereby reducethe damage on the crew, is now being generalized. Particularly, anairbag for a driver seat installed within the steering wheel, or anairbag for a passenger seat installed within the instrument panel isbeing included as standard equipment. Furthermore, recently, in additionto them, to protect the crew at lateral collision, an automobile havinga side airbag in the seat or a curtain type airbag for covering a sidewindow has been increased.

Traditionally, a type using gunpowder for generating gas has been mainlyused as the airbag. However, recently, from a viewpoint of recyclingefficiency or environmental consciousness, a type in which inert gassuch as argon is charged into an inflator under high pressure has becomemainly used instead of using the gunpowder. In this type, since theinert gas must be kept to be always under high pressure, the inflatormust have sufficiently high strength.

Generally, the airbag inflator is produced by a processing steel pipe.In the inert gas charged type airbag, since the inert gas is charged inthe inflator under high pressure, from a viewpoint of reliability of theseam, a seamless steel pipe is mostly used as the pipe for the inflator.Typically, the seamless steel pipe is subjected to cold drawing processto have a predetermined size and cut into a predetermined length, thenboth pipe ends are processed by pressing and sealing plates are weldedto the pipe ends, thereby products (inflator) are formed.

From the situation, as the steel pipe for the inflator, a seamless steelpipe having sufficient strength and toughness, excellent formability,and excellent weldability is desired. For such demand, for example,JP-A-10-140283 proposes a method for manufacturing a high-strength,high-toughness steel pipe for the airbag inflator, in which the steelcontaining 0.01 to 0.20% of C, 0.50% or less of Si, 0.30 to 2.00% of Mn,0.020% or less of P, 0.020% or less of S, and 0.10% or less of Al, orfurther containing at least one of 0.50% or less of Mo, 0.10% or less ofV, 0.50% or less of Ni, 1.00% or less of Cr, 0.50% or less of Cu, 0.10%or less of Ti, 0.10% or less of Nb, and 0.005% or less of B, and Fe andunavoidable impurities as residue is used to manufacture a seamlesssteel pipe. The seamless steel pipe is remained as cold working, orsubjected to normalizing, normalizing, or quenching and tempering afterthe cold working.

JP-A-10-140249 proposes a method for producing a high-strength,high-toughness steel pipe for the airbag inflator, wherein the steelhaving a same composition as the composition described in JP-A-10-140283is used to manufacture a steel pipe. The steel pipe is normalized at 850to 1000° C., and then subjected to the cold working to have apredetermined size, or subjected to the normalization, or the quenchingand tempering after a stress relief normalizing. JP-A-10-140250 proposesa method for manufacturing a high-strength, high-toughness steel pipefor the airbag inflator, wherein the steel having a same composition asthe composition described in JP-A-10-140283 is used to manufacture asteel pipe. The steel pipe is quenched at 850 to 1000° C., or furthertempered at 450° C. or more and less than A_(c1) transformation point,then subjected to the cold working to have a predetermined size andremained as it is, or subjected to the normalizing after the coldworking.

It is described that a high-strength, high-toughness steel pipe for theairbag having high dimension accuracy, excellent formability, andweldability, and a tensile strength of 590 N/mm² or more can bemanufactured according to the technique described in JP-A-10-140283,JP-A-10-140249, or JP-A-10-140250.

DISCLOSURE OF THE INVENTION

Recently, there has been a demand for miniaturization and weight savingof airbag system, and there is a demand for further increase of strengthas the seamless steel pipe for the airbag inflator. Particularly, forthe curtain type airbag, a large volume of gas is required such that theairbag can cover front and rear side-windows, in addition, a chargingpressure of 50 MPa or more is required. To meet such requirements, aseamless steel pipe having 900 MPa or more of tensile strength isdesired as the inflator. The seamless steel pipe should have 900 MPa ormore of tensile strength finally after being subjected to the colddrawing or heat treatment.

In the technique described in JP-A-10-140283, JP-A-10-140249, orJP-A-10-140250, which aims to manufacture a 590 MPa class,high-strength, seamless steel pipe, there is a problem that thetechnique cannot meet the demand for further increase of strengthdesired for the steel pipe for the inflator.

The invention aims to solve the problem in the traditional artadvantageously, and propose a method for manufacturing a seamless steelpipe with high strength, high toughness, and high formability, the pipehaving excellent formability and weldability in manufacturing of theinflator, in addition, a high tensile strength of 900 MPa or more, andhigh toughness or ductility exhibited in a drop weight test at −60° C.for a halved steel pipe.

The inventors have earnestly made a study on various factors effected onthe strength, toughness, and formability to overcome the problem.Consequently, as methods for manufacturing the seamless steel pipe, twomethods below were found. (1) A seamless steel pipe having a steelcomposition containing a reduced content of C and proper amounts of Crand Mo is manufactured. The seamless steel pipe is subjected to the colddrawing. Then the pipe is subjected to the quenching and tempering, orthe normalizing. (2) A seamless steel pipe having a steel compositioncontaining the reduced amount of C content and the proper amounts of Crand Mo is manufactured. The seamless steel pipe is subjected to thequenching and tempering, or the normalizing. Then the pipe is subjectedto the cold drawing. It was found that by either of the methods (1),(2), increase of the strength can be designed, particularly, a seamlesssteel pipe with small decrease in circumferential strength and smallanisotropy is obtained.

The invention was based on the findings and completed through furtherinvestigations.

That is, the invention is summarized as follows.

-   -   (1) A method for manufacturing a seamless steel pipe with the        high strength, high toughness, and high formability for the        airbag characterized by comprising a process for manufacturing a        raw material for steel pipe having a composition containing 0.01        to 0.10% of C, 0.5% or less of Si, 0.10 to 2.00% of Mn, more        than 1.0% and 2.0% or less of Cr, and 0.5% or less of Mo by mass        to form the seamless steel pipe, a quenching and tempering        process for heating the seamless steel pipe at a temperature in        a range of A_(c3) transformation point or more and 1050° C. or        less and then quenching, then tempering at a temperature in a        range of 450° C. or more and the A_(c1) transformation point or        less, and a process for drawing the seamless steel pipe in cold        working.    -   (2) The method for manufacturing the seamless steel pipe with        the high strength, high toughness, and high formability for the        airbag in (1), wherein order of the cold drawing process and the        quenching and tempering process is reversed.    -   (3) The method for manufacturing the seamless steel pipe with        the high strength, high toughness, and high formability for the        airbag in (1) or (2) characterized in that the composition        further contains, in addition to the above composition, one or        two or more selected from 1.0% or less of Cu, 1.0% or less of        Ni, 0.10% or less of Nb, 0.10% or less of V, 0.10% or less of        Ti, and 0.005% or less of B by mass.    -   (4) The method for manufacturing the seamless steel pipe with        the high strength, high toughness, and high formability for the        airbag in (1), (2), or (3) characterized in that the seamless        steel pipe is subjected to the normalizing for heating the pipe        to a temperature in a range from 850 to 1000° C. and then        air-cooling, instead of the quenching and tempering process.

BEST MODE FOR CARRYING OUT THE INVENTION

First, the reasons for setting the limit for the components of the rawmaterial for steel pipe for use is described. Hereinafter, “% by mass”in the composition is simply shown as “%”. C: 0.01to 0.10%

C is an element that contributes to increase of the strength of steel.However, excessive C content of more than 0.10% causes decrease informability and weldability. On the other hand, when the C content isless than 0.01%, a desired tensile strength is hard to be ensured.Therefore, in the invention, C is limited within a range from 0.01 to0.10%. Preferably, the C content is 0.03 to 0.08%. Si: 0.5% or less

Si is an element that increases the strength of steel, and preferablycontained at 0.1% or more to obtain such an effect. However, sinceexcessively large content of Si causes decrease in ductility andformability, the Si content was limited to 0.5% or less in theinvention. Preferably, the Si content is 0.1 to 0.4%. Mn: 0.10to 2.00%

Mn is an element that improves the strength, and must be contained at0.10% or more to ensure a desired strength in the invention. On theother hand, when 2.00% of Mn is contained, the ductility is decreased,in addition, the formability and weldability are decreased. Therefore,Mn was limited to 2.00% or less. Preferably, the Mn content is 1.00 to1.70%. Cr: more than 1.0% and 2.0% or less

Cr is an effective element for improving the strength and corrosionresistance of steel, and must be contained at more than 1.0% mainly forensuring a high strength in the invention. On the other hand, when morethan 2.0% of Cr is contained, the ductility is decreased, in addition,the formability, weldability, and toughness are decreased. Therefore, Crwas limited within a range of more than 1.0% and 2.0% or less.Preferably, the Cr content is 1.1 to 1.5%. Mo: 0.5% or less

Mo is an element that increases the strength of steel and improves thequenching characteristics, and preferably contained at 0.1% or more inthe invention. On the other hand, when more than 0.5% of Mo iscontained, the ductility is decreased, and weld crack resistance islowered. Therefore, Mo was limited to 0.5% or less. Preferably, the Mocontent is 0.3% or less.

In the invention, in addition to the basic composition, one or two ormore selected from 1.0% or less of Cu, 1.0% or less of Ni, 0.10% or lessof Nb, 0.10% or less of V, 0.10% or less of Ti, and 0.005% or less of Bcan be further contained.

Each of Cu, Ni, Nb, V, Ti, and B acts to increase the strength, and oneor two or more of them can be selectively contained as needed.

Cu is an element that increases the strength of steel, in addition,improves corrosion resistance. However, when more than 1.0% of Cu iscontained, hot working characteristics are lowered. Therefore, Cu ispreferably limited to 1.0% or less. More preferably, the Cu content is0.5% or less.

Ni is an element that increases the strength of steel, and improves thequenching characteristics and the toughness. However, since Ni isexpensive, it is preferable that Ni is limited to 1.0% or less in theinvention. More preferably, the Ni content is 0.5% or less.

Nb is an element that increases the strength of steel throughprecipitation hardening, and improves the toughness by refiningmicrostructure. However, when more than 0. 10% of Ni is contained, thetoughness is conversely decreased. Therefore, Nb is preferably limitedto 0.10% or less. More preferably, the Nb content is 0.01 to 0.05%.

V is an element that increases the strength of steel throughprecipitation hardening, and improves the quenching characteristics.However, when more than 0.10% of V is contained, the toughness isdecreased. Therefore, V is preferably limited to 0.10% or less. Morepreferably, the V content is 0.01 to 0.05%.

Ti is an element that increases the strength of steel throughprecipitation hardening, and improves the toughness by refining themicrostructure. However, when more than 0.10% of Ti is contained, thetoughness is conversely decreased. Therefore, Ti is preferably limitedto 0.10% or less. More preferably, the Ti content is 0.005 to 0.03%.

B is an element that contributes to increase of the strength throughimprovement of the quenching characteristics. However, when more than0.005% of B is contained, the toughness is decreased. Therefore, B ispreferably limited to 0.005% or less. More preferably, the B content is0.0005 to 0.002%.

The residue other than the above components is Fe and the avoidableimpurities. As the avoidable impurities, 0.03% or less of P, 0.01% orless of S, and 0.10% or less of Al are allowed.

It is preferable that molten steel having the above composition isproduced using a known steel making process such as a converter or anelectric furnace, and then preferably made into a raw material for steelpipe such as billet using a known casting process such as a continuouscasting process or an ingot making process. The slab, which is producedusing the continuous casting process, can be made into the billet byrolling.

Then, the resultant raw material for steel pipe is manufactured using atypical manufacturing process of Mannesmann-plug mill method orMannesmann-mandrel mill method, thereby a seamless steel pipe is formed.The manufacturing process of the seamless steel pipe may include othermethod than the above methods. The manufactured seamless steel pipe issubjected to either one of the following two treatments. (1) After thequenching and tempering, or the normalizing, the cold drawing isperformed. (2) After the cold drawing, the quenching and tempering, orthe normalizing is performed.

The cold drawing can be performed using a generally known cold drawingapparatus without requiring a particular apparatus. Although conditionsof the cold drawing is not needed to be particularly limited as long asa predetermined size of pipe can be formed, it is preferable from a viewof ensuring dimension accuracy to adjust the diameter reductionpercentage to be within a range from 5 to 25% and the thicknessreduction percentage to be within a range from 10 to 30%.

Heating temperature for quenching is set to be a temperature in a rangefrom A_(c3) transformation point to 1050° C. When the heatingtemperature is less than A_(c3) transformation point, uniformaustenization cannot be achieved. On the other hand, when the heatingtemperature is high, over 1050° C., crystal grains become coarse andthus the toughness is decreased. Therefore, the heating temperature forthe quenching was set to be 1050° C. or less in the invention. Afterheating at a temperature within the above range, cooling is performed bywater cooling (quenching) to form a quenching microstructure (martensitemicrostructure). Preferably, the heating temperature for quenching isthe A_(c3) transformation point or more and 950° C. or less.

The tempering is performed at a temperature within a range of 450° C. ormore and A_(c1) transformation point or less. The tempering temperatureis preferably selected to be a temperature at which the strength,toughness, and formability are best together. When the temperingtemperature is less than 450° C., the tempering is inadequate, and thusa desired toughness cannot be obtained. On the other hand, when thetemperature exceeds the A_(c1) transformation point, the quenchingmicrostructure cannot be obtained, and the strength is decreased,thereby a desired strength cannot be ensured. Therefore, the temperingtemperature was limited to a temperature in a range of 450° C. or moreand the Ac, transformation point or less. Preferably, the temperature is500 to 700° C. Cooling after the tempering is preferably performed at arate of air cooling rate or more.

In the normalizing, heating is performed at a temperature within a rangefrom 850 to 1000° C. and then air-cooling is performed. When thenormalizing temperature is less than 850° C., austenite grains cannot beunified adequately. On the other hand, when the normalizing temperatureexceeds 1000° C., the crystal grains become coarse and thus a desiredtoughness is hardly ensured. Therefore, the normalizing temperature ispreferably limited to 850 to 1000° C. Preferably, the normalizingtemperature is 850 to 950° C.

The seamless steel pipe subjected to the quenching and tempering or thenormalizing is then preferably subjected to descaling by acid pickling,or bend straightening as needed, thereby pipe products (steel pipe) areformed.

The seamless steel pipe manufactured by the above method has a highstrength of 900 MPa in tensile strength and a high toughness orductility exhibited in the drop weight test at −60° C. for the halvedsteel pipe, and is formed into a steel pipe having excellent formabilityand weldability, thereby a steel pipe suitable for the inflator for thecurtain type airbag is formed.

EXAMPLE

A raw material for steel pipe (billet with a diameter of 140 mm) havinga composition shown in Table 1 was heated to 1250° C., and formed into aseamless steel pipe (outer diameter of 34.0 mm and thickness of 3.2 mm,or outer diameter of 38.1 mm and thickness of 3.3 mm) by theMannesmann-mandrel mill method. They are piercing, mandrel mill rolling,and reducer rolling.

The seamless steel pipes were subjected to the quenching and temperingor the normalizing under conditions shown in Table 2. Then, each of theseamless steel pipes after the heat treatments was subjected to the colddrawing in a diameter reduction percentage of 11.8% or 8.9%, and athickness reduction percentage of 21.9% or 18.2% under the conditionsshown in Table 2, thereby a pipe product with the diameter of 30.0 mmand the thickness of 2.5 mm or the diameter of 34.7 mm and the thicknessof 2.7 mm was formed.

The seamless steel pipes were subjected to the cold drawing in diameterreduction percentage of 11.8% or 8.9%, and thickness reductionpercentage of 21.9% or 18.2% under conditions shown in Table 3, therebya steel pipe with the diameter of 30.0 mm and the thickness of 2.5 mm orthe diameter of 34.7 mm and the thickness of 2.7 mm was formed. Then,the steel pipes were subjected to the quenching and tempering or thenormalizing under the conditions shown in Table 2. Then, the seamlesssteel pipes after the heat treatments were straightened to remove thebend, thereby the pipe products were formed.

Test pieces were sampled from the resulting pipe products, and a tensiletest was carried out, thereby longitudinal tensile characteristics wereexamined. The tensile test was carried out in accordance with JIS Z 2241standard after sampling No. 11 test pieces (tubular test piece) definedby JIS Z 2201. Furthermore, a hydraulic burst test was achieved, andcircumferential strength was converted from the burst pressure.

For the resultant pipe products, the drop weight test was carried out at−60° C., and the toughness was examined. The drop weight test at −60° C.was performed in such a way that the pipe products were semicircularlyhalved, then a test of dropping a weight of 100 kgf from a height of 500mm onto the pipes was carried out at −60° C. After the test, fractureswere observed and occurrence of brittle failure was examined. The testwas set to be repeated three times, and it was determined that a casethat no brittle failure occurred in the three tests was O, a case thatthe brittle failure occurred in all tests was X, and other cases were Δ.

For the resultant pipe products, the pipe ends were contracted to haveouter diameters of 20 mm and 25 mm using spinning machining, then cracksin the machined portion were observed, and thus the formability wasevaluated. It was determined that in a case that no crack was observed,the formability was O, in a case that a crack was observed, theformability was X.

After contracting the pipe end to have the outer diameter of 20 mm usingthe spinning machining, the pipe end was welded with sealing plates, andafter the welding, occurrence of cracks was examined visually and with amicroscope, and thus the weldability was evaluated.

The obtained results are shown in Table 2 and Table 3. Each of theexamples of the invention is a seamless steel pipe having a tensilestrength of 900 MPa or more and high toughness, and having an excellentformability, in addition, an excellent weldability. On the other hand,in a comparative example outside the scope of the invention, the tensilestrength is less than 900 MPa, the toughness is decreased, or theformability is decreased. In the comparative example, sufficientproperties as the steel pipe for the inflator for the curtain typeairbag are not obtained.

INDUSTRIAL APPLICABILITY

As above, according to the invention, a seamless steel pipe having highdimension accuracy, in addition, high strength, high toughness, and highformability can be stably manufactured, thereby industrially remarkableadvantages are provided. TABLE 1 steel chemical composition (% by mass)No. C Si Mn P S Cr Mo Al Cu Ni V Nb Tl B remarks A 0.05 0.25 1.45 0.0180.003 1.40 0.22 0.018 Ex. of the Invention B 0.07 0.39 1.67 0.015 0.0021.21 0.19 0.024 Ex. of the Invention C 0.04 0.18 1.81 0.014 0.002 1.250.25 0.031 0.25 0.12 Ex. of the Invention D 0.06 0.28 1.38 0.016 0.0021.36 0.31 0.034 0.29 Ex. of the Invention E 0.05 0.31 1.41 0.015 0.0031.24 0.27 0.029 0.035 Ex. of the Invention F 0.06 0.33 1.58 0.018 0.0021.27 0.34 0.028 0.024 0.031 Ex. of the Invention G 0.08 0.39 1.88 0.0140.003 1.73 0.31 0.026 0.031 0.023 0.008 Ex. of the Invention H 0.03 0.401.65 0.017 0.002 1.65 0.32 0.021 0.030 0.0015 Ex. of the Invention I0.06 0.29 1.51 0.016 0.002 1.31 0.29 0.030 0.0013 0.0012 Ex. of theInvention J 0.06 0.20 1.32 0.016 0.004 1.25 0.20 0.027 0.18 0.10 0.0330.021 0.0011 Ex. of the Invention K 0.12 0.25 0.85 0.014 0.003 1.49 0.310.033 Comp. Ex. L 0.07 0.74 1.54 0.013 0.003 1.67 0.28 0.029 Comp. Ex. M0.06 0.33 0.02 0.014 0.002 1.56 0.27 0.028 Comp. Ex. N 0.06 0.27 2.340.013 0.002 1.43 0.32 0.031 Comp. Ex. O 0.06 0.30 1.28 0.014 0.003 0.850.21 0.029 Comp. Ex. P 0.08 0.27 1.43 0.015 0.002 2.28 0.33 0.028 Comp.Ex. Q 0.07 0.34 1.59 0.017 0.003 1.29 0.74 0.031 Comp. Ex.

TABLE 2 depression heat treatment steel pipe size heat treatmentconditions steel pipe size percentage in conditions after before drawingbefore drawing after drawing cold drawing drawing steel outer thick-quenching tempering normalizing outer thick- diameter thicknessquenching tempering pipe steel diameter ness temperature temperaturetemperature diameter ness reduction reduction temperature temperatureNo. No. (mm) (mm) (° C.) (° C.) (° C.) (mm) (mm) (%) (%) (° C.) (° C.) 1A 34.0 3.2 920 600 30.0 2.5 11.8 21.9 2 34.0 3.2 30.0 2.5 11.8 21.9 4 B38.1 3.3 930 580 34.7 2.7 8.9 18.2 5 C 38.1 3.3 920 550 34.7 2.7 8.918.2 6 38.1 3.3 920 34.7 2.7 8.9 18.2 7 D 34.0 3.2 920 580 30.0 2.5 11.821.9 8 E 38.1 3.3 920 600 34.7 2.7 8.9 18.2 9 F 38.1 3.3 920 600 34.72.7 8.9 18.2 10 G 34.0 3.2 950 30.0 2.5 11.8 21.9 11 34.0 3.2 920 63030.0 2.5 11.8 21.9 12 H 34.0 3.2 920 500 30.0 2.5 11.8 21.9 13 I 34.03.2 920 480 30.0 2.5 11.8 21.9 14 J 38.1 3.3 920 460 34.7 2.7 8.9 18.215 K 34.0 3.2 920 600 30.0 2.5 11.8 21.9 16 L 34.0 3.2 920 600 30.0 2.511.8 21.9 17 M 38.1 3.3 920 450 34.7 2.7 8.9 18.2 18 N 38.1 3.3 920 65034.7 2,7 8.9 18.2 19 O 38.1 3.3 920 460 34.7 2.7 8.9 18.2 20 P 38.1 3.3920 630 34.7 2.7 8.9 18.2 21 Q 38.1 3.3 920 600 34.7 2.7 8.9 18.2characteristics of seamless steel pipe products steel circumferentialpipe longitudinal tension property tension property No. TS (MPa) El (%)TS (MPa) Toughness Machinability Weldability remarks 1 1025 14.3 965 ∘ ∘∘ Ex. of the Invention 2 842 16.8 748 ∘ ∘ ∘ Comp. Ex. 4 1068 15.8 946 ∘∘ ∘ Ex. of the Invention 5 1063 14.9 937 ∘ ∘ ∘ Invention in theApplication 6 1116 12.0 978 x x ∘ Comp. Ex. 7 1087 14.6 968 ∘ ∘ ∘ Ex. ofthe Invention 8 1132 15.2 1002 ∘ ∘ ∘ Ex. of the Invention 9 1066 14.3954 ∘ ∘ ∘ Ex. of the Invention 10 1039 15.5 923 ∘ ∘ ∘ Ex. of theInvention 11 1023 14.9 938 ∘ ∘ ∘ Ex. of the Invention 12 1028 14.8 938 ∘∘ ∘ Ex. of the Invention 13 1086 15.4 965 ∘ ∘ ∘ Ex. of the Invention 141073 13.9 926 ∘ ∘ ∘ Ex. of the Invention 15 1099 11.8 957 x x x Comp.Ex. 16 1058 12.0 938 x x x Comp. Ex. 17 991 15.6 833 ∘ ∘ ∘ Comp. Ex. 181069 11.6 965 x x x Comp. Ex. 19 986 14.9 823 ∘ ∘ ∘ Comp. Ex. 20 110611.6 972 x x x Comp. Ex. 21 1139 12.1 989 x x x Comp. Ex.

TABLE 3 steel pipe size depression steel pipe size after cold percentagein cold heat treatment conditions after before drawing drawing drawingcold drawing outer outer diameter thickness quenching temperingnormalizing steel pipe steel diameter thickness diameter thicknessreduction reduction temperature temperature temperature No. No. (mm)(mm) (mm) (mm) (%) (%) (° C.) (° C.) (° C.) 22 A 34.0 3.2 30.0 2.5 11.821.9 920 500 23 34.0 3.2 30.0 2.5 11.8 21.9 24 34.0 3.2 30.0 2.5 11.821.9 920 600 25 B 38.1 3.3 34.7 2.7 8.9 18.2 930 500 26 C 38.1 3.3 34.72.7 8.9 18.2 920 470 27 38.1 3.3 34.7 2.7 8.9 18.2 920 28 D 34.0 3.230.0 2.5 11.8 21.9 920 500 29 E 38.1 3.3 34.7 2.7 8.9 18.2 920 530 30 F38.1 3.3 34.7 2.7 8.9 18.2 920 520 31 G 34.0 3.2 30.0 2.5 11.8 21.9 95032 34.0 3.2 30.0 2.5 11.8 21.9 920 550 33 H 34.0 3.2 30.0 2.5 11.8 21.9920 460 34 I 34.0 3.2 30.0 2.5 11.8 21.9 920 450 35 J 38.1 3.3 34.7 2.78.9 18.2 920 450 36 K 34.0 3.2 30.0 2.5 11.8 21.9 920 550 37 L 34.0 3.230.0 2.5 11.8 21.9 920 550 38 M 38.1 3.3 34.7 2.7 8.9 18.2 920 450 39 N38.1 3.3 34.7 2.7 8.9 18.2 920 600 40 O 38.1 3.3 34.7 2.7 8.9 18.2 920460 41 P 38.1 3.3 34.7 2.7 8.9 18.2 920 600 42 Q 38.1 3.3 34.7 2.7 8.918.2 920 600 characteristics of seamless steel pipe productslongitudinal circumferential steel pipe tension properly tensionproperty No. TS (MPa) El (%) TS (MPa) toughness machinabilityweldability remarks 22 1041 14.7 1033 ∘ ∘ ∘ Ex. of the Invention 23 84016.8 748 ∘ ∘ ∘ Comp. Ex. 24 987 13.8 983 ∘ ∘ ∘ Ex. of the Invention 25942 15.3 938 ∘ ∘ ∘ Invention in the Application 26 963 15.2 952 ∘ ∘ ∘Comp. Ex. 27 1237 11.7 1210 x x ∘ Ex. of the Invention 28 1025 14.5 1025∘ ∘ ∘ Ex. of the Invention 29 1128 14.2 1132 ∘ ∘ ∘ Ex. of the Invention30 1055 13.6 1049 ∘ ∘ ∘ Ex. of the Invention 31 923 14.8 918 ∘ ∘ ∘ Ex.of the Invention 32 1065 14.0 1066 ∘ ∘ ∘ Ex. of the Invention 33 96414.8 965 ∘ ∘ ∘ Ex. of the Invention 34 934 14.8 965 ∘ ∘ ∘ Ex. of theInvention 35 921 15.2 918 ∘ ∘ ∘ Comp. Ex. 36 1051 12.3 1049 x x x Comp.Ex. 37 1021 12.5 1023 x x x Comp. Ex. 38 802 15.9 800 ∘ ∘ ∘ Comp. Ex. 391099 11.2 1093 x x x Comp. Ex. 40 783 15.9 780 ∘ ∘ ∘ Comp. Ex. 41 105612.3 1055 x x x Comp. Ex. 42 988 13.5 989 x x x Comp. Ex.

1. A method for manufacturing a seamless steel pipe having highstrength, high toughness, and high formability for an airbagcharacterized by comprising, a process for manufacturing a steel pipehaving a composition containing 0.01 to 0.10% of C, 0.5% or less of Si,0.10 to 2.00% of Mn, more than 1.0% and 2.0% or less of Cr, and 0.5% orless of Mo by mass to form the seamless steel pipe, a quenching andtempering process for heating the seamless steel pipe to a temperaturein a range of A_(c3) transformation point or more and 1050° C. or lessand then quenching, then tempering at a temperature in a range of 450°C. or more and A_(c1) transformation point or less, and a cold drawingprocess for drawing the seamless steel pipe in cold working.
 2. Themethod for manufacturing the seamless steel pipe having the highstrength, high toughness, and high formability for the airbag in claim1, wherein order of the cold drawing process and the quenching andtempering process is reversed.
 3. The method for manufacturing theseamless steel pipe having the high strength, high toughness, and highformability for the airbag according to claim 1 and 2 characterized inthat the composition further contains, at least one element selectedfrom 1.0% or less of Cu, 1.0% or less of Ni, 0.10% or less of Nb, 0.10%or less of V, 0.10% or less of Ti, and 0.005% or less of B.
 4. Themethod for manufacturing the seamless steel pipe having the highstrength, high toughness, and high formability for the airbag accordingto claim 3 characterized in that the seamless steel pipe is subjected tonormalizing, for heating to a temperature in a range from 850 to 1000°C. and then air cooling, instead of the quenching and tempering process.